1. Field of the Invention
This invention relates to laundry dryers where ambient air moves past various types of heating apparatus, passes through a tumbler containing moisture-laden laundry, and accomplishes drying of the laundry by extracting the moisture therefrom. More particularly, the invention relates to the recovery of the heat contained within the air exhausted from the laundry dryer, enabling the otherwise wasted heat energy to be returned to the dryer's heating apparatus so that significant fuel savings are realized.
2. Description of the Prior Art
There have been numerous attempts to recycle exhausted hot air from laundry dryers to reduce overall energy consumption. The majority of these have employed stationary passive heat exchangers. See in particular, U.S. Pat. No. 4,063,590, entitled "Preheater for Clothes Dryer", issued to C. McConnell. A similar arrangement, utilizing a shell-and-tube type passive heat exchanger, instead of a folded plate type, is revealed in U.S. Pat. No. 3,859,735, issued to Katterjohn. There are two fundamental problems inherent in application of stationary exchangers to processes of this type. The first is that of lint accumulation within the heat exchanger baffles, causing an increase in heat exchanger airflow resistance and a corresponding decrease in thermal efficiency. Additionally, the requirement for extremely large amounts of heat transfer surface to provide meaningful thermal efficiency results in severe impact upon the cost of the system and its capability for offsetting equipment cost through fuel savings. In contrast, the regenerative capability of rotary heat exchange media as well as its inherent ability to provide large amounts of transfer surface in a limited space renders it most useful for applications such as laundry drying. Inherent also in laminar-flow media rotary heat exchangers is "self-cleaning" of the heat transfer surface afforded by the reversal of fluid flow as the exchanger rotates between counterflowing airstreams. These factors, combined with others which will become evident later in this disclosure, provide for operation of laundry dryers which is substantially more energy-efficient than has heretofore been the case.
The principles and usage of rotary heat exchangers are well documented. See, in particular, U.S. Pat. Nos. 2,579,912; 2,887,456; 3,290,764; 3,702,156; 4,093,435; 4,307,774. Such exchangers, commonly referred to as "heat recovery wheels", have traditionally been confined to high-volume, energy-intensive airflow situations, owing to the cost of the heat exchangers and associated installations. Recent increases in fuel costs relative to costs of other goods now make application of such exchangers economically feasible on systems which utilize smaller volumes.
A problem which has traditionally plagued rotary heat exchangers of the type disclosed in the prior art concerns effective sealing between the rotating exchanger and its adjacent stationary supporting structure. Inefficient sealing has resulted in loss of thermal effectiveness due to air bypassing the heat exchange media at the exchanger's periphery; additional problems have resulted from leakage at the divider between counterflowing fluid streams, often causing what is referred to as "cross-contamination" of the new, incoming fluid stream by the contaminated exhaust stream. This ineffective sealing is due to the difficulty associated with the economical manufacture of a large diameter circular matrix which remains planar upon rotation. The problem is enhanced when the heat exchanger is subjected to temperatures in excess of 300.degree. F. ("industrial process" applications), in which the heat exchanger undergoes severe radial as well as planar distortion due to cyclical exposure of the heat exchange media to temperature gradients of the counterflowing airstreams. To overcome the leakage, various "floating" contact seals have been developed. See, in particular, U.S. Pat. No. 4,068,708, issued to Y. Sakaki. While this solves the leakage problem, its cost is prohibitive for lower-efficiency, less cost-effective exchangers used at "environmental" temperatures (as for heating and air-conditioning). The sealing/leakage problem reduces to that of manufacturing a "flat" (planar with respect to its rotating axis) heat exchanger or providing economical means of adjustment whereby the exchanger's "flatness", as well as concentricity, can be controlled. Means for achieving this adjustment are presented in detail in this disclosure.
In view of the foregoing information, it is obvious that the principles of rotary heat exchange are known in the art. The invention described herein utilizes known principles of said heat exchange, combining them with mechanical and structural improvements which markedly improve the fuel-efficiency of laundry dryers, as well as improving the thermal effectiveness and operation of the exchangers themselves.